Theoretical studies on Ru-catalyzed pauson-khand-type [2+2+1] and related [2+2+1+1] cycloadditions

Density functional theory (DFT) calculations have been carried out to understand the mechanism of the Ru₃(CO)₁₂-catalyzed Pauson-Khand-type [2+2+1] reaction and related [2+2+1+1] cycloadditions. The geometries were optimized using the BP86/6-31G*(SDD for Ru) method, and the energies were evaluated with the 6-311+G*(SDD) basis set. We found that these reactions are initiated by a CO-alkyne coupling, forming a ruthenacyclobutenone intermediate, and the widely accepted alkene-alkyne coupling pathway has a much higher activation energy. In the intermolecular reaction between alkene and alkyne, the formation of quinones and hydroquinones through [2+2+1+1] cycloadditions is more favorable than the Pauson-Khand-type reaction, while the intramolecular reaction with 1,6-enyne leads to a favorable Pauson-Khand-type reaction. These results are in agreement with experimental observations. For the [2+2+1+1] cycloadditions we found that the formation of quinones is favored over the formation of hydroquinones due to the preferred insertion of alkynes, which can be attributed to the preferred orbital interaction between the π orbital of the alkyne moiety and the d orbital of the metal center.